Cyclopolymerization of Bis(diazocarbonyl) Compounds Leading to

Article pubs.acs.org/Macromolecules

Cyclopolymerization of Bis(diazocarbonyl) Compounds Leading to Well-Defined Polymers Essentially Consisting of Cyclic Constitutional Units Hiroaki Shimomoto, Misaki Kikuchi, Junya Aoyama, Dai Sakayoshi, Tomomichi Itoh, and Eiji Ihara* Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan S Supporting Information *

ABSTRACT: Cyclopolymerization of bis(diazocarbonyl) compounds is described for the first time. By choosing appropriate reaction conditions and monomer structures, the cyclopolymerization of bis(diazoacetate)s efficiently proceeded to give carbon−carbon main chain cyclopolymers with well-defined structures, as confirmed by MALDI-TOF-MS analyses. The products are a new kind of cyclopolymer with respect to polymer structure: there exists no free methylene in the sp3-carbon-based polymer backbone in contrast to conventional cyclopolymers prepared from divinyl compounds, which inevitably have free methylenes in and between cyclic units in their polymer backbones. The resulting cyclopolymers with closely aligned cyclic units along the polymer backbone showed a much higher glass transition temperature compared to the corresponding polymers without cyclized repeating units.

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INTRODUCTION Polymers containing consecutive cyclic units in the polymer backbone have attracted attention due to their unique properties and potential functions. As for the synthesis of such polymers, cyclopolymerization of bifunctional monomers is a direct and convenient method, where two reactive groups in the monomers are polymerized through alternating reactions of intermolecular addition and intramolecular cyclizing addition.1−3 Up to now, various nonconjugated dienes or divinyl compounds have been employed as bifunctional monomers, providing sp3−sp3 carbon−carbon main chain cyclopolymers with repeating cyclic units (Scheme 1A). For example, the cyclopolymerization of 1,5-hexadiene and 1,6-heptadiene with early transition metal complexes including Ti, Zr, and Hf affords hydrocarbon polymers with five- and/or six-membered cyclic units along the main chain.4−17 In addition, the complexes of late transition metals, such as Pd, Co, and Fe, have been reported to catalyze the cyclopolymerization of nonconjugated dienes with polar functional groups in high stereoregularity.18,19 Other examples of cyclopolymerization affording sp3−sp3 carbon−carbon main chain cyclopolymers include radical or ionic polymerization of difunctional styrenic compounds,20−23 di(meth)acrylates,24−29 divinyl ethers,30,31 and bis(acrylamide)s.32,33 This strategy can afford various functionalized cyclopolymers with larger rings, although elabolate design of bifunctional monomers is required to overcome the difficulty of the thermodynamically unfavorable large ring formations. In addition to vinyl polymerization, transition-metal-initiated polymerization of alkyl or aryl diazoacetates (N2CHCO2R) is a useful method for preparing sp3−sp3 carbon−carbon main © XXXX American Chemical Society

Scheme 1. Cyclopolymerization of Nonconjugated Dienes or Divinyl Compounds (A) and Bis(diazoacetate)s (B)

chain polymers.34,35 In this method, the polymer backbone is constructed from one carbon unit with N2 release in each chaingrowth step, giving polymers possessing an ester substituent on every main chain carbon atom. It has been demonstrated that the resulting polymers show unique properties compared to the corresponding vinyl polymer counterparts, i.e., poly(meth)Received: August 19, 2016 Revised: October 7, 2016

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Table 1. Cyclopolymerization of Bis(diazoacetate)s Using πAllylPdCl/Borate Initiating Systemsa

acrylates, due to densely accumulated functional groups around the polymer backbone.36−40 It occurred to us that cyclopolymerization of bis(diazoacetate)s would allow us to prepare a new type of cyclopolymers with respect to polymer structure (Scheme 1B), where there exists no free methylene in and between cyclic units in the backbone, unlike the conventional cyclopolymers prepared from nonconjugated dienes or divinyl compounds mentioned above. The resulting cyclopolymers are expected to provide unique properties and functions due to closely aligned cyclic units derived from the absence of free methylenes in the main chains. Actually, the reported synthesis for this type of polymer backbone structure has been limited to the radical polymerization of N-substituted maleimides,41 and thus, successful cyclopolymerization of bis(diazoacetate)s in our attempts could establish the first general methodology for the synthesis of the unique polymer structure, which can be regarded as “ring-expanded poly(maleic anhydride)”. Herein, we will describe the results of the cyclopolymerization of bis(diazoacetate)s using Pd complexes and the investigation of thermal properties of the resulting cyclopolymers.

run

monomer

monomer conc (M)

borateb

yieldc (%)

Mnd

Mw/Mnd

1 2 3 4 5 6 7 8 9 10 11 12 13

Bin-0 Bin-0 Bin-0 Bin-1 Bin-1 Bin-1 Bin-2 Bin-2 Bin-2 Cy-1 Cy-1 Ph-1 Ph-1

0.24 0.16 0.06 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

none none none none NaBPh4 LiBPh4 none NaBPh4 LiBPh4 none LiBPh4 none LiBPh4

41 58 56 46 75 68 45 72 76 40 63 50 54

30000 23300 3000 3100 4400 3800 4800 6300 5200 2800 3700 2600 4500

14.8 1.98 1.92 1.47 1.84 1.60 1.27 1.70 1.98 1.51 1.55 1.64 1.93

a

Polymerization conditions: [Pd] = 2[(π-allylPdCl)2], [monomer]/ [Pd] = 25, in THF at 0 °C for 15 h. b[NaBPh4] or [LiBPh4]/[Pd] = 1.2−2.5. cAfter purification with preparative SEC. dDetermined by SEC (PMMA standards).

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RESULTS AND DISCUSSION Bis(diazoacetate) Monomers for Cyclopolymerization. As a bifunctional monomer, bis(diazoacetate)s in which two reactive diazocarbonyl groups are attached to a linker through oxyethylene spacers were designed (Scheme 2), where various Scheme 2. Cyclopolymerization of Bis(diazoacetate)s Using π-AllylPdCl/Borate Initiating Systems

Figure 1. SEC profiles of the products obtained by cyclopolymerization of Bin-0 under various monomer concentrations [(A) 0.24 M (run 1 in Table 1), (B) 0.16 M (run 2 in Table 1), and (C) 0.06 M (run 3 in Table 1)].

functional groups can be introduced as a linker and the ring size can be tailored by changing the oxyethylene unit lengths. In this study, binaphthylene (Bin), cyclohexylene (Cy), and phenylene (Ph) groups were chosen as a linker (R), and the oxyethylene repeating unit lengths (x) were changed from zere to two. Polymerization of Binaphthylene-Linked Monomers (Bin-0, Bin-1, and Bin-2). In general, cyclopolymerization is conducted at a diluted monomer concentration to prevent an undesired intermolecular cross-linking reaction. Accordingly, to find an appropriate concentration for the bis(diazoacetate)s, we first conducted the reaction using Bin-0 under various monomer concentrations (runs 1−3 in Table 1). Figure 1 shows size exclusion chromatography (SEC) profiles of the resulting polymers. At the monomer concentration of 0.24 M, which is equal to or lower than a typical concentration for the polymerization of “monofunctional” diazoacetates such as ethyl and benzyl diaozoacetates, the reaction of Bin-0 was conducted with a feed ratio of [Bin-0]/[Pd] = 25 in THF using the πallylPdCl initiating system, giving a polymer with multimodal

and very broad molecular weight distribution (Figure 1A, run 1 in Table 1). This result indicates that efficient cyclopolymerization does not occur due to intermolecular crosslinking reaction. Although the value of Mw/Mn became much lower at the monomer concentration of 0.16 M, the SEC curve was still bimodal (Figure 1B, run 2 in Table 1). At an even lower concentration of 0.06 M, a polymer with unimodal molecular weight distribution and much lower Mn was obtained (Figure 1C, run 3 in Table 1). The polymer structure (run 3) was analyzed by MALDITOF-MS and NMR spectrometry (Figures S1 and S2). In the MS spectrum, we could not observe a set of signals with an interval of the m/z value (366) corresponding to the repeating unit derived from the ring-closing reaction of Bin-0 B

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still exist small broad cluster peaks which would be derived from an undesired intermolecular cross-linking reaction. The comparison of these MS spectra reveals that the addition of borates as a cocatalyst was effective in increasing not only yield and Mn but also intramolecular cyclization efficiency.42 On the basis of our previous work,43 each peak cluster can be assigned to a Na+ adduct of the cyclopolymer with Cl derived from πallylPdCl and CH2CH2CH2CH2OH derived from THF used as solvent at α- and ω-chain ends, respectively; the observed m/z value with n = 7 (m/z = 3313.4) agrees with the calculated one (m/z = 3312.9). A plausible initiation and termination mechanism will be described later. Figure 3 shows 1H NMR spectra of the polymers obtained by π-allylPdCl alone (run 4) and π-allylPdCl/LiBPh4 (run 6) along with that of the monomer Bin-1. In the spectra of polymers, the signal at 4.4 ppm for a proton of the diazocarbonyl group of Bin-1 disappears. Instead, the signal for methine protons attached to the main chain carbons appears as a broad peak at 3−4 ppm overlapping with other signals, and the broad signals assignable to binaphthylene and oxyethylene groups appear as expected for the formation of cyclopolymers. However, in the spectrum of the product obtained by π-allylPdCl alone, sharp signals with small intensities were observed at similar positions as those of monomer peaks along with the broad signals. In addition, there exists an unexpected peak at 6.2 ppm; it can be assigned to the protons of the cis-CHCH− unit, which is formed by intermolecular CC forming coupling reaction of two diazocarbonyl groups with elimination of two N2 molecules38,44 to give cross-linking or branched products, while the peak almost completely disappears in the spectra of products prepared by the π-allylPdCl/LiBPh4 initiating system. These observations also support that the presence of LiBPh4 is effective to give cyclopolymers with high cyclizing efficiency. When Bin-2 with longer oxyethylene spacers was used as a bis(diazoacetate), the polymerization under a diluted concentration gave a polymer with unimodal molecular weight distribution, and the addition of borates was effective in increasing yield and Mn (runs 7−9 in Table 1), in a similar manner as with Bin-1. The structures of the products were analyzed by MALDI-TOF-MS and NMR spectroscopy (Figures S6 and S7). In the MALDI-TOF-MS spectra, although a set of signals which corresponds to the cyclopolymer with the same chain ends as Bin-1 was observed, broad cluster peaks became more significant, which should be attributed to the difficulty of the thermodynamically unfavorable 22-membered-ring formations. With these results of cyclopolymerization using binaphthylene-linked monomers, we can conclude that Bin-1 with a moderate spacer length (x = 1) is the best monomer among binaphthylene-linked monomers for efficient cyclopolymerization of bis(diazoacetate)s. However, quantitative cyclization efficiency was not achieved, as demonstrated in the MS spectra, where there still exist small broad cluster peaks assignable to an undesired intermolecular cross-linking reaction. The imperfect cyclization could be ascribed to the free rotation of binaphthylene linker, which is unfavorable for highly efficient cyclization. Cyclopolymerization of Cy-1 and Ph-1: Synthesis of Well-Defined Cyclopolymers. Next, in order to fix the two diazocarbonyl groups in positions favorable for cyclopolymerization, Cy-1 and Ph-1 bearing cyclohexylene and phenylene linkers, respectively, were employed as bis(diazoacetate)s, and

accompanied by N2 release. Instead, the broad cluster peaks appear, suggesting a randomly cross-linked ill-defined structure of the product because of insufficient cyclization. This undesired polymer structures can be attributed to unfavorable 10-membered ring formation expected for cyclopolymerization of Bin-0. To find an appropriate design of monomers for efficient cyclopolymerization, the reaction using binaphthylene-linked diazoacetates with different spacer lengths was conducted. First, we employed Bin-1, in which an oxyethylene unit is introduced between binaphthylene linker and each diazocarbonyl group, for the cyclopolymerization. We expected the introduction of the flexible oxyethylene spacers would facilitate the ring-closing, although the larger 16-membered-ring formation would still be thermodynamically unfavorable. The cyclopolymerization of Bin-1 was carried out under a diluted monomer concentration (0.05 M) as well, giving a polymer with unimodal molecular weight distribution (Figure S3, run 4 in Table 1). The addition of borates as a cocatalyst was found to be effective in increasing yield and Mn, as is the case with the polymerization of oligo(ethylene glycol)-containing diazoacetates reported in our previous publication;39 the cyclopolymerization of Bin-1 using π-allylPdCl/NaBPh4 and π-allylPdCl/LiBPh4 initiating systems gave polymers with Mn = 4400 in 75% yield and Mn = 3800 in 68% yield, respectively, while π-allylPdCl alone gave a polymer with Mn = 3100 in 46% yield (runs 4−6 in Table 1). However, the Mn values are still lower than the theoretical ones calculated from the monomer feed ratio and yield. This is attributed to an undesirable termination reaction mentioned later in this article. Figure 2 shows MALDI-TOF-MS spectra (linear mode) of the obtained polymers from Bin-1 (runs 4 and 6). In contrast

Figure 2. MALDI-TOF-MS spectra (linear mode) of the products obtained by cyclopolymerization of Bin-1 using (A) π-allylPdCl alone (run 4 in Table 1) and (B) π-allylPdCl/LiBPh4 (run 6 in Table 1).

to the spectrum of the product obtained from Bin-0, we can observe several sets of sharp signals with an interval of the m/z value (454) corresponding to the repeating unit derived from the ring-closing reaction of Bin-1 accompanied by N2 release. In particular, in the MS spectrum of the product obtained by the π-allylPdCl/LiBPh4 initiating system (Figure 2B), one set of signals was predominantly observed, indicating efficient cyclopolymerization proceeds through 16-membered-ring formation to give a cyclopolymer with uniform end groups, although there C

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Figure 3. 1H NMR spectra of (A) Bin-1 and (B) products obtained by cyclopolymerization of Bin-1 using π-allylPdCl alone (upper, run 4 in Table 1) and π-allylPdCl/LiBPh4 (lower, run 6 in Table 1).

the polymerization results are listed in Table 1 (runs 10−13). As observed in the cyclopolymerization of bis(diazoacetate)s with binaphthylene linker, the addition of borates as a cocatalyst was effective to increase yield and Mn. Figures 4A and 4B show MALDI-TOF-MS spectra (linear mode) of the polymers obtained from Cy-1 (run 11) and Ph-1 (run 13), respectively, using π-allylPdCl/LiBPh4. Noteworthy is that broad cluster peaks became remarkably small, especially in the MS spectrum of the product obtained from Ph-1, suggesting

high cyclization efficiency. In addition, in both spectra, we can clearly observe a main set of signals, whose interval between peak clusters correspond to the molecular weight of the repeating unit derived from Cy-1 (m/z = 284) or Ph-1 (m/z = 278). Accordingly, by fixing the two diazocarbonyl groups in positions favorable for cyclopolymerization, we have successfully synthesized cyclopolymers with well-defined structures. Each peak cluster can be assignable to a Na+ adduct of the polymer with Cl and CH2CH2CH2CH2OH at α- and ω-chain ends, respectively, as is the case with the polymers obtained from Bin-1 and Bin-2, although there is a slight discrepancy between the observed mass values and the calculated ones due to measurements in linear mode with low resolution. In order to confirm the polymer end groups more accurately, we carried out MALDI-TOF-MS measurements in reflector mode with high resolution, and the spectrum for cyclopoly(Ph-1)′ is shown in Figure 5A. Similarly, we can clearly observe a main set of signals with an interval of the repeating unit (m/z = 278) with a higher resolution. Figure 5B shows a peak cluster around m/z = 2080 and the simulated one with the above-mentioned chain ends (Cl and CH2CH2CH2CH2OH at α- and ω-chain ends, respectively) with n = 7, where we can observe almost complete agreement between the observed appearance and the simulated one. On the basis of the results of chain-end-structure analyses, we can propose a plausible initiation and termination mechanism of cyclopolymerization of bis(diazoacetate)s using π-allylPdCl/ borate initiating systems (Scheme 3).45 In our previous publication,43 we revealed that products obtained by polymerization of ethyl diazoacetate (EDA) using π-allyPdCl/NaBPh4 initiating system had a Ph group at their α-chain ends, derived from the initiation from Ph-Pd species generated by transmetalation of Ph-B to the Pd. However, MALDI-TOF-MS analyses in this study indicated that the α-chain end of the cyclopolymers was Cl, even though the same π-allyPdCl/ NaBPh4 system was employed. Accordingly, we assume that the real initiating species for the polymerization is a Pd ate complex

Figure 4. MALDI-TOF-MS spectra (linear mode) of (A) cyclopoly(Cy-1)′ (run 11 in Table 1) and (B) cyclopoly(Ph-1)′ (run 13 in Table 1). D

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of higher Mn polymers but produced cyclopolymers with uniform end groups. Thermal Properties of Cyclopolymers. Finally, the thermal properties of the cyclopolymers were evaluated by differential scanning calorimetry (DSC). Here, cyclopoly(Ph1)′ was employed for the measurements because the cyclization efficiency was highest among the polymers synthesized in this study. For comparison, poly(Ph-1)′ with a similar side chain but no cyclic units was prepared and subjected to the measurement as well. As shown in Figure 6, the glass transition

Figure 6. DSC thermograms of poly(Ph-1)′ (Mn = 8300, Mw/Mn = 1.48) and cyclopoly(Ph-1)′ (Mn = 7200, Mw/Mn = 1.50). The latter was prepared by SEC fractionation of cyclopoly(Ph-1)′ with Mn = 4500 (run 13 in Table 1) to adjust the molecular weight of the samples to a comparable degree.

Figure 5. (A) MALDI-TOF-MS spectra (reflector mode) of (A) cyclopoly(Ph-1)′ (run 13 in Table 1) and (B) a peak cluster around m/z = 2080 and the simulated one with Cl and CH2CH2CH2CH2OH at α- and ω-chain ends, respectively.

temperature (Tg) of cyclopoly(Ph-1)′ (Mn = 7200, Mw/Mn = 1.50) was 81 °C, which was much higher than that of poly(Ph1)′ with a similar Mn and Mw/Mn values (11 °C; Mn = 8300, Mw/Mn = 1.48), revealing that the introduction of cyclic repeating units into the polymer backbone effectively increased Tg values.

[PdPhCl]− generated by the transmetalation bearing Cl and Ph groups, both of which have potential to nucleophilically attack a diazo-bearing carbon atom in diazoacetates. Although the reason is not clear at present, not Ph but Cl on the ate complex initiated the polymerization of the bis(diazoacetate)s employed in this study. As for the ω-chain end structure, at the end of polymerization, the propagating chain end nucleophilically attacks a coordinated THF used as solvent, giving OH chain end after ring-opening and protonolysis with an acidic quencher, as we reported as one of the observed termination mechanisms in the EDA polymerization with this initiator.43 This undesirable termination reaction prevented the formation

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CONCLUSIONS We have demonstrated that bis(diazocarbonyl) compounds can be used as monomer for cyclopolymerization for the first time. By choosing appropriate reaction conditions and linker structures and spacer lengths of bifunctional monomers, the cyclopolymerization of bis(diazoacetate)s using Pd complexes efficiently proceeded to give a new type of cyclopolymer with well-defined structure, in which there exists no free methylene

Scheme 3. A Plausible Initiation and Termination Mechanism of Cyclopolymerization of Bis(diazoacetate)s Using π-AllylPdCl/ Borate Initiating Systems

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in and between cyclic units in the polymer backbone. The resulting cyclopolymers with closely aligned cyclic units along the polymer backbone showed a much higher glass transition temperature compared to the corresponding polymer without cyclized repeating units. Further studies such as cyclopolymerization of optically active bis(diazoacetate)s for syntheses of new types of chiral polymers6,25,46 are underway in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01809. SEC profiles, NMR spectra, and MALDI-TOF-MS spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone and Fax: +81-89-927-8547; e-mail: [email protected]. jp (E.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (JSPS KAKENHI Grant 15H00755) and “Studying the Function of Soft Molecular Systems by the Concerted Use of Theory and Experiment (No. 2503)” (JSPS KAKENHI Grants 26104525 and 16H00841), a Grant-in-Aid for Scientific Research (C) (JSPS KAKENHI Grant 15K05521), and a Grant-in-Aid for Young Scientists (B) (JSPS KAKENHI Grant 16K17916). The authors thank Applied Protein Research Laboratory in Ehime University for its assistance in NMR and MALDI-TOF-MS measurements and Advanced Research Support Center in Ehime University for its assistance in elemental analysis.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b01809 Macromolecules XXXX, XXX, XXX−XXX

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(42) It appears that LiBPh4 was more effective as a cocatalyst than NaBPh4 in increasing cyclization efficiency [see MALDI-TOF-MS (Figure S4) and 1H NMR (Figure S5) spectra of the product obtained by cyclopolymerization of Bin-1 using the π-allylPdCl/NaBPh4 initiating system], and this trend prevails for other monomers than Bin-1. At the moment, the reason for the difference is not clear. Further investigations are needed to demonstrate this point. (43) Ihara, E.; Akazawa, M.; Itoh, T.; Fujii, M.; Yamashita, K.; Inoue, K.; Itoh, T.; Shimomoto, H. π-AllylPdCl-Based Initiating Systems for Polymerization of Alkyl Diazoacetates: Initiation and Termination Mechanism Based on Analysis of Polymer Chain End Structures. Macromolecules 2012, 45, 6869−6877. (44) The protons of trans-CHCH− unit should exist at around at 6.8 ppm but overlap with aromatic protons derived from binaphthylene linker. (45) As for the propagation reaction, we can consider two types of mechanisms: either direct nucleophilic attack of the growing chain end to the monomer coordinated with the Pd or N2 elimination to form a carbene complex and subsequent carbene migratory insertion into the growing chain end. Although it is not clear at present which mechanism is involved in the propagation reaction, we tentatively assume the former in Scheme 3. (46) Kakuchi, T.; Obata, M. Synthesis and Mechanism of a MainChain Chiral Polymer Based on Asymmetric Cyclopolymerization. Macromol. Rapid Commun. 2002, 23, 395−406.

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DOI: 10.1021/acs.macromol.6b01809 Macromolecules XXXX, XXX, XXX−XXX